Niobium for capacitor and capacitor using sintered body of the niobium

ABSTRACT

1. A niobium powder for capacitors, wherein the chromium content is 50 ppm by mass or less, granulated product and sintered body thereof, and producing method of those; 2. a capacitor constructed by one part electrode formed of the niobium sintered body, another part electrode and a dielectric material interposed between two electrodes, and its producing method; and 3. an electronic circuit and electronic device using the capacitor. A capacitor having good voltage resistance properties can be manufactured by using the niobium sintered body for capacitors of the present invention, wherein the chromium content is 50 ppm by mass or less.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This is an application based on the prescription of 35 U.S.C. Section111(a) with claiming the benefit of filing date of U.S. Provisionalapplication Ser. No. 60/277,280 filed Mar. 21, 2001 under the provisionof 35 U.S.C. 111(b), pursuant to 35 U.S.C. Section 119(e) (1).

TECHNICAL FIELD

The present invention relates to niobium (a niobium powder, aniobium-granulated product and a sintered body produced therefrom),which can provide a capacitor having good voltage resistancecharacteristics and a large capacitance per unit volume, and alsorelates to a capacitor using the sintered body.

BACKGROUND ART

Capacitors for use in electronic instruments such as portable telephoneand personal computer are demanded to have a small size and a largecapacitance. Among conventional capacitors, tantalum capacitors arepreferred because of their large capacitance for the size and goodperformance. In these tantalum capacitors, a sintered body of tantalumpowder is generally used for the anode moiety. In order to increase thecapacitance of the tantalum capacitor, it is necessary to increase theweight of the sintered body or to use a sintered body increased in thesurface area by pulverizing the tantalum powder.

The former method of increasing the weight of the sintered bodynecessarily involves enlargement of the capacitor shape and cannotsatisfy the requirement for downsizing. On the other hand, in the lattermethod of pulverizing the tantalum powder to increase the surface area,the pore size of the tantalum sintered body decreases or closed poresincrease at the stage of sintering and therefore, impregnation of thecathode agent in the later process becomes difficult. As one of meansfor solving these problems, a capacitor using a sintered body of powderof a material having a dielectric constant larger than that of tantalumis being studied. The materials having a larger dielectric constantinclude niobium.

Niobium is homologous to tantalum but is greatly different from tantalumin the characteristics as the material for capacitors. For example, iftantalum contains 10,000 ppm by mass of oxygen as impurities, theleakage current characteristics are greatly deteriorated, however,niobium has no such a problem and even if niobium has an oxygen contentof tens of thousands of ppm by mass, the leakage current characteristicsare scarcely deteriorated.

Capacitors manufactured using niobium as a raw material are, however,inferior in the voltage resistance characteristics to capacitorsmanufactured using tantalum as a raw material.

Known publications describing the relationship between the amount ofimpurity elements contained in the niobium powder and the capacitorperformance include International Patent Publications WO00/49633 andWO00/56486. The former discloses that the capacitor performance such asspecific leakage current of the capacitor can be improved by reducingthe content of specific impurity elements such as iron, nickel andcobalt, to 100 ppm by mass or less, and the latter discloses that thiseffect can be attained by adjusting the carbon content to from 40 to 200ppm by mass and the iron, nickel and chromium content to approximatelyfrom 5 to 200 ppm by mass. However, either publication does not disclosethe relation between the chromium content and the voltage resistancecharacteristics of the capacitor.

DISCLOSURE OF THE INVENTION

As a result of extensive investigations on the niobium used as the rawmaterial of the capacitor which is improved in the voltage resistancecharacteristics, the present inventors have found that a correlationgenerally exists between the voltage resistance of a capacitor and thecontent of impurities in the niobium (B, C, F, Na, Mg, Ca, Fe, Ni, Zn,W, Cr and others), particularly between the voltage resistance and thechromium content. And they found that a capacitor using niobium reducedin the chromium content (particularly 50 ppm by mass or less) isremarkably improved in the voltage resistance. This improvement isconsidered to be attributable to that the deterioration in the vicinityof impurity elements partially present in the dielectric layer of acapacitor is outstanding particularly in the vicinity of chromiumelement. However, niobium raw materials usually available have a largechromium content and therefore, if the raw material is used as it is,the above-described property cannot be attained. The present inventorshave found a method for producing niobium reduced in the chromiumcontent, which is used as a raw material for capacitors having a smallsize and good voltage resistance characteristics. The present inventionhas been accomplished based on this finding.

The present invention relates to a niobium for capacitors, a niobiumpowder, a granulated product and a sintered body thereof, a capacitorusing the sintered body, and producing method thereof in below:

1. a niobium for capacitors, mainly comprising niobium characterized inthat the chromium content is 50 ppm by mass or less;

2. the niobium for capacitors mainly comprising niobium as described in1 above, which contains a niobium nitride;

3. the niobium for capacitors mainly comprising niobium as described in1 above, which contains a niobium carbide;

4. the niobium for capacitors mainly comprising niobium as described in1 above, which contains a niobium boride;

5. the niobium for capacitors mainly comprising niobium as described in1 above, which is a powder having an average particle size of 0.1 μm to3 μm;

6. the niobium for capacitors mainly comprising niobium, as described in1 above, which is a a niobium-granulated product having an averageparticle size of 10 μm to 300 μm;

7. the niobium for capacitors mainly comprising niobium as described in1 above, which is a niobium sintered body having a BET specific surfacearea of from 0.5 m²/g to 7 m²/g;

8. a capacitor constructed by one electrode formed of a niobium sinteredbody mainly comprising niobium, the other electrode and a dielectricmaterial interposed between the two electrodes, wherein the sinteredbody is a sintered body of the niobium for capacitors described in anyone of 1 to 6 above;

9. a capacitor constructed by one electrode formed of a niobium sinteredbody mainly comprising niobium, the other electrode and a dielectricmaterial interposed between the two electrodes, wherein the sinteredbody is a sintered body of the niobium for capacitors described in 7above;

10. the capacitor as described in 8 or 9 above, wherein the maincomponent of the dielectric material constituting the capacitor is aniobium oxide;

11. the capacitor as described in any one of 8 to 10 above, wherein theother electrode is at least one member selected from the groupconsisting of an electrolytic solution; an organic semiconductor and aninorganic semiconductor;

12. the capacitor as described in 11 above, wherein the organicsemiconductor is at least one organic semiconductor selected from thegroup consisting of an organic semiconductor comprising a benzopyrrolinetetramer and chloranile, an organic semiconductor mainly comprisingtetrathiotetracene, an organic semiconductor mainly comprisingtetracyanoquinodimethane and an organic semiconductor mainly comprisingan electrically conducting polymer obtained by doping a dopant into apolymer comprising two or more repeating units represented by thefollowing formula (1) or (2):

(wherein R₁ to R₄ each represents a monovalent group selected from thegroup consisting of a hydrogen atom, a linear or branched, saturated orunsaturated alkyl, alkoxy or alkylester group having from 1 to 10 carbonatoms, a halogen atom, a nitro group, a cyano group, a primary,secondary or tertiary amino group, a CF₃ group, a phenyl group and asubstituted phenyl group; the hydrocarbon chains in each of the pairs R₁and R₂, and R₃ and R₄ may combine at an arbitrary position to form adivalent chain for forming at least one 3-, 4-, 5-, 6- or 7-memberedsaturated or unsaturated hydrocarbon cyclic structure together with thecarbon atoms substituted by R₁ and R₂ or by R₃ and R₄; the cyclic bondedchain may contain a bond of carbonyl, ether, ester, amide, sulfide,sulfinyl, sulfonyl or imino at an arbitrary position; X represents anoxygen atom, a sulfur atom or a nitrogen atom; R₅ is present only when Xis a nitrogen atom and each R₅ independently represents a hydrogen atomor a linear or branched, saturated or unsaturated alkyl group havingfrom 1 to 10 carbon atoms);

13. the capacitor as described in 12 above, wherein the electricallyconducting polymer is an electrically conducting polymer comprising arepeating unit represented by the following formula (3):

(wherein R₆ and R₇ each independently represents a hydrogen atom, alinear or branched, saturated or unsaturated alkyl group having from 1to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or7-membered saturated hydrocarbon cyclic structure containing two oxygenelements resulting from those alkyl groups combining with each other atan arbitrary position, and the cyclic structure includes a structurehaving a vinylene bond which may be substituted, and a phenylenestructure which may be substituted);

14. the capacitor as described in 11 above, wherein the organicsemiconductor is at least one member selected from the group consistingof polypyrrole, polythiophene, polyaniline and substitution derivativesthereof;

15. the capacitor as described in 11 above, wherein the organic orinorganic semiconductor has an electrical conductivity of 10⁻² S·cm⁻¹ to10³ S·cm⁻¹;

16. a method for producing a niobium for capacitors characterized incomprising, in the production process thereof, a step of reducing thechromium content of a substance mainly comprising niobium;

17. the method for producing a niobium for capacitors as described in 16above, wherein the step of reducing the chromium content is a step oftreating a substance mainly comprising niobium with a solutioncontaining at least one acid selected from the group consisting of ahydrofluoric acid, a nitric acid, a sulfuric acid and a hydrochloricacid;

18. the method for producing a niobium for capacitors as described in 16or 17 above, wherein the substance mainly comprising niobium contains aniobium nitride;

19. the method for producing a niobium for capacitors as described in 16or 17, wherein the substance mainly comprising niobium contains aniobium carbide;

20. the method for producing a niobium for capacitors as described in 16or 17 above, wherein the substance mainly comprising niobium contains aniobium boride;

21. the method for producing a niobium for capacitors as described in 16above, wherein the substance mainly comprising niobium is a powder;

22. the method for producing a niobium for capacitors as described in 21above, wherein the niobium powder has an average particle size of 0.1 μmto 3 μm;

23. the method for producing a niobium for capacitors as described in 16above, wherein the substance mainly comprising niobium is a niobiumgranulated product having an average particle size of 10 μm to 300 μm;

24. the method for producing a niobium for capacitors as described in 16above, wherein the substance mainly comprising niobium is a niobiumsintered body having a BET specific surface area of 0.5 m²/g to 7 m²/g;

25. a method for producing a niobium granulated product for capacitors,which is characterized in granulating the niobium powder for capacitorsdescribed in 5 above;

26. a method for producing a niobium sintered body for capacitors, whichis characterized in sintering the niobium granulated product forcapacitors described in 6 above;

27. a niobium for capacitors obtained by the method described in any oneof 16 to 22 above;

28. a niobium granulated product for capacitors obtained by the methoddescribed in 25 above;

29. a niobium sintered body for capacitors obtained by the methoddescribed in 26 above;

30. a method for producing a capacitor constructed by one electrodemainly comprising niobium, the other electrode and a dielectric materialinterposed between the two electrodes, which is characterized incomprising a step of reducing the chromium content in the electrodemainly comprising niobium in the production process of the capacitor;

31. a method for producing a capacitor constructed by one electrodeformed of a niobium sintered body mainly comprising niobium, the otherelectrode and a dielectric material interposed between the twoelectrodes, which is characterized in comprising the method forproducing a niobium for capacitors described in at least one of 16 to 22above as a production process;

32. a method for producing a capacitor constructed by one electrodeformed of a niobium sintered body mainly comprising niobium, the otherelectrode and a dielectric material interposed between the twoelectrodes, which is characterized in comprising the method forproducing a niobium granulated product for capacitors described in 25above as a production process;

33. a method for producing a capacitor constructed by one electrodeformed of a niobium sintered body mainly comprising niobium, the otherelectrode and a dielectric material interposed between the twoelectrodes, which is characterized in comprising the method forproducing a niobium sintered body for capacitors described in 26 aboveas a production process;

34. a method for producing the capacitor described in 10 above, whereinthe niobium oxide is formed by electrolytic oxidation;

35. a capacitor obtained by the production method described in any oneof 30 to 33 above;

36. an electronic circuit using the capacitor described in any one of 8to 15 and 35 above;

37. an electronic instrument using the capacitor described in any one of8 to 15 and 35 above;

38. The niobium for capacitors as described in 1 above, wherein thechromium content is 40 ppm by mass or less;

39. The niobium for capacitors as described in 1 above, wherein thechromium content is 5 ppm by mass or less; and

40. The niobium for capacitors as described in 1 above, wherein thechromium content is 3 ppm by mass or less.

MODE FOR CARRYING OUT THE INVENTION

One embodiment for obtaining the niobium for capacitors of the presentinvention is described below based on one of the embodiment examples.

The niobium for capacitors of the present invention is a substance whichmainly comprises niobium and can be used as a material for producing acapacitor. In this embodiment, a powder, a granulated product and asintered body are included.

A niobium powder as the raw material of the niobium for capacitors canbe obtained, for example, by reducing niobium halide with hydrogen,magnesium or sodium, reducing potassium niobium fluoride with sodium,electrolyzing potassium niobium fluoride with a molten salt (NaCl+KCl)on a: nickel cathode, or introducing hydrogen into a metal niobium ingotand then pulverizing the product. The niobium powder obtained by thesemethods is considered to contain impurities from niobium raw material,reducing agent and the environment of the instrument used.

Chromium in the niobium is probably intermingled as an impurity throughsuch a route. The niobium powder, the niobium granulated product and theniobium sintered body of the present invention can be obtained byreducing the chromium content in a niobium powder, a niobium granulatedproduct and a niobium sintered body to 50 ppm by mass or less,preferably 40 ppm by mass or less, more preferably 5 ppm by mass or lessand much more preferably 3 ppm by mass or less.

For obtaining a niobium having a small chromium content, a method ofusing a raw material having a sufficiently small chromium content andusing a particular niobium production apparatus designed to preventintermingling of even a slight amount of chromium, and a method ofproviding a step of removing chromium intermingled on the way of theprocess of producing the niobium, may be used. Insofar as the chromiumcontent can be reduced to 50 ppm by mass or less, any method can beapplied to the present invention without particular limitation.

Examples thereof include a method of using a niobium raw material or areducing agent each having a higher purity and preventing theintermingling of chromium by using an instrument free of chromium, and amethod of washing the above-described niobium powder by using an acidcontaining at least one acid of a hydrofluoric acid, a nitric acid, asulfuric acid and a hydrochloric acid, and an alkali, or by using theabove-described acid, an alkali and a hydrogen peroxide in sequence orin combination.

Preferred is the latter method using an acid and a hydrogen peroxide.This method can be applied also to a niobium adjusted to have acomposition as the niobium for capacitors (namely, a niobium containingniobium nitride, which is described later) or a niobium having adjustedin a shape (namely, a powder, a granulated product or a sintered body).Since this method can be used in a relatively later step during theproduction of a niobium for capacitors, it is not necessary in manypreceding steps to use raw materials or apparatuses particularlydesigned to prevent the intermingling of chromium.

The niobium powder of the present invention preferably has an averageparticle size of 3 μm or less so as to increase the specific surfacearea of the powder, because the capacitance of a capacitor producedusing the niobium powder is in a proportional relation with the specificsurface area of the powder. In this regard, it is effective forelevating the capacitance of a capacitor to [more] increase the surfacearea, that is, to reduce the average particle size. However, if theparticle size is too small, the impregnation of cathode agent in thelater step becomes difficult. On taking account of the balancetherebetween, the average particle size of the niobium powder ispreferably from 0.1 μm to 3 μm. The average particle size of the niobiumgranulated product is preferably from 10 μm to 300 μm.

The niobium granulated product of the present invention can be obtained,for example, by granulating the niobium powder to an appropriate size.For the granulation, a conventionally known method can be employed.Examples thereof include a method where powder particles are leftstanding at a high temperature of 500° C. to 2,000° C. in a vacuum andthen wet or dry cracked, a method where powder particles are mixed withan appropriate binder such as acrylic resin or polyvinyl alcohol andthen cracked, and a method where powder particles are mixed with anacrylic resin or an appropriate compound such as camphor, phosphoricacid or boric acid, left standing at a high temperature in a vacuum andthen wet or dry cracked. The particle size of the niobium granulatedproduct can be freely changed by the degree of granulation and cracking,however, a niobium granulated product having an average particle size of10 μm to 300 μm is usually used. The niobium granulated product for usemay be classified after the granulation and cracking. Also, the niobiumgranulated product after the granulation may be mixed with anappropriate amount of powder particles before the granulation and used(in the present invention, a granulated product [having] mixed therewithnon-granulated powder particles is also referred to as “a granulatedproduct”). Or niobium granulated products having different averageparticle sizes in an appropriate amount may be mixed to use. Thespecific surface area of the thus-produced niobium granulated productcan be freely changed, and a niobium granulated product having thespecific surface area from 0.5 m²/g to 7 m²/g is usually used.

In the niobium powder of the present invention, a part of niobium may bebonded with at least one of nitrogen, carbon and boron so as to improvethe leakage current characteristics. The niobium powder may comprise anyof niobium nitride, niobium carbide and niobium boride, which are thecompounds of nitrogen, carbon and boron, respectively, or may comprise acombination of two or three of these compounds. The sum total of theirbonded amounts, that is, the total content of nitrogen, carbon and boronvaries depending on the shape of the niobium powder and, in the case ofa powder having an average particle size of approximately from 0.1 μm to3 μm, the total content is from 50 to 200,000 ppm by mass, preferablyfrom 300 to 20,000 ppm by mass. If the total content is less than 50 ppmby mass, the improvement of the leakage current characteristics is notenough, whereas if it exceeds 200,000 ppm by mass, the capacitancecharacteristics are deteriorated.

The nitridation for forming a niobium nitride may be performed by anyone of liquid nitridation, ion nitridation and gas nitridation or by acombination of these methods. Among these, gas nitridation in a nitrogengas atmosphere is preferred because this treatment is simple and easy.The gas nitridation in a nitrogen gas atmosphere can be performed byallowing the niobium powder to stand in a nitrogen gas atmosphere. Bythe nitridation at an atmosphere temperature of 2,000° C. or less for astanding time of several hours or less, a niobium powder having anobjective nitrided amount can be obtained. As the treatment is performedat a higher temperature, the nitridation can be completed within ashorter time. As such, the nitrided amount can be managed by controllingthe nitridation temperature and the nitridation time.

The carbonization for forming a niobium carbide may also be performed byany one of gas carbonization, solid-phase carbonization and liquidcarbonization or by a combination of these methods. For example, thecarbonization can be performed by allowing a niobium powder to standtogether with a carbon material or an organic material containingcarbon, such as methane, at 2,000° C. or less under reduced pressure forseveral minutes to tens of hours.

The boronization for forming a niobium boride may be performed by eithergas boronization or solid-phase boronization. The boronization can beperformed, for example, by allowing a niobium powder to stand togetherwith boron pellet or a boron source such as boron halide (e.g.,trifluoroboron) at 2,000° C. or less under reduced pressure for severalminutes to tens of hours.

The niobium sintered body for capacitors of the present invention can beproduced by sintering the above-described niobium powder or granulatedproduct. One example of the production method therefor is describedbelow, however, the present invention is by no means limited to thisexample.

The sintered body may be obtained, for example, by press-molding theniobium powder into a predetermined shape and then heating it at 500° C.to 2,000° C. for several minutes to several hours under a reducedpressure of 10⁻⁴ Pa to 10² Pa or in an inert gas such as Ar.

It is also possible to prepare a lead wire comprising a valve-actingmetal such as niobium or tantalum and having an appropriate shape and anappropriate length and integrally mold the lead wire at thepress-molding of the niobium powder such that a part of the lead wire isinserted into the inside of the molded article, thereby designing thelead wire to work out to a leading line of the sintered body. Thespecific surface area of the thus-produced niobium sintered body of thepresent invention can be freely changed, and a niobium sintered bodyhaving a specific surface area of 0.5 m²/g to 7 m²/g is usually used.

Using the thus-produced sintered body as one electrode, a capacitor canbe manufactured by interposing a dielectric material between thiselectrode and the other electrode. Examples of the dielectric materialof the capacitor include a dielectric material comprising niobium oxide.The dielectric material comprising niobium oxide can be obtained, forexample, by chemically forming the niobium sintered body as oneelectrode in an electrolytic solution. For chemically forming theniobium electrode in an electrolytic solution, an aqueous protonic acidsolution is generally used, such as an aqueous 0.1% phosphoric acidsolution or an aqueous sulfuric acid solution. In the case of obtainingthe dielectric material comprising niobium oxide by chemically formingthe niobium electrode in an electrolytic solution, the capacitor of thepresent invention is an electrolytic capacitor and the niobium sideserves as an anode.

On the other hand, in the capacitor of the present invention, the otherelectrode is not particularly limited and for example, at least onecompound selected from electrolytic solutions, organic semiconductorsand inorganic semiconductors known in the art of aluminum electrolyticcapacitor, may be used.

Specific examples of the electrolytic solution include adimethylformamide-ethylene glycol mixed solution having dissolvedtherein 5% by mass of an isobutyl-tripropylammonium tetrafluoroborateelectrolyte, and a propylene carbonate-ethylene glycol mixed solutionhaving dissolved therein 7% by mass of tetraethylammoniumtetrafluoroborate electrolyte.

When the organic or inorganic semiconductor used has an electricalconductivity of 10⁻² S·cm⁻¹ to 10³ S·cm⁻¹, the produced capacitor canhave a smaller impedance value and this is preferred. Specific examplesof the organic semiconductor which can give such characteristics includean organic semiconductor comprising a benzenepyrroline tetramer andchloranile, an organic semiconductor mainly comprisingtetrathiotetracene, an organic semiconductor mainly comprisingtetracyanoquinodimethane, and an organic semiconductor mainly comprisingan electrically conducting polymer obtained by doping a dopant into apolymer comprising a repeating unit represented by following formula (1)or (2):

Wherein R₁ to R₄ each independently represents a monovalent groupselected from the group consisting of a hydrogen atom, a linear orbranched, saturated or unsaturated alkyl, alkoxy or alkylester grouphaving from 1 to 10 carbon atoms, a halogen atom, a nitro group, a cyanogroup., a primary, secondary or tertiary amino group, a CF₃ group, aphenyl group and a substituted phenyl group; the hydrocarbon chains ineach of the pairs R₁ and R₂, and R₃ and R₄ may combine at an arbitraryposition to form a divalent chain for forming at least one 3-, 4-, 5-,6- or 7-membered saturated or unsaturated hydrocarbon cyclic structuretogether with the carbon atoms substituted by R₁ and R₂ or by R₃ and R₄;the cyclic bonded chain may contain a bond of carbonyl, ether, ester,amide, sulfide, sulfinyl, sulfonyl or imino at an arbitrary position; Xrepresents an oxygen atom, a sulfur atom or a nitrogen atoms R₅ ispresent only when X is a nitrogen atom and independently representshydrogen or a linear or branched, saturated or unsaturated alkyl grouphaving from 1 to 10 carbon atoms.

In the present invention, R₁ to R₄ of formula (1) or (2) eachindependently preferably represents a hydrogen atom or a linear orbranched, saturated or unsaturated alkyl or alkoxy group having from 1to 6 carbon atoms, and each of the pairs R₁ and R₂, and R₃ and R₄ maycombine with each other to form a ring.

In the present invention, the electrically conducting polymer comprisinga repeating unit represented by formula (1) is preferably anelectrically conducting polymer comprising a structure unit representedby the following formula (3) as a repeating unit:

wherein R₆ and R₇ each independently represents a hydrogen atom, alinear or branched, saturated or unsaturated alkyl group having from 1to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or7-membered saturated hydrocarbon cyclic structure containing two oxygenelements resulting from the alkyl groups combining with each other at anarbitrary position; and the cyclic structure includes a structure havinga vinylene bond which may be substituted, and a phenylene structurewhich may be substituted.

The electrically conducting polymer containing such a chemical structurehas a polaron or bipolaron within the molecule and therefore, iselectrically charged. This polymer is doped with a dopant and for thedopant, known dopants can be used without limitation.

Specific examples of the inorganic semiconductor include an inorganicsemiconductor mainly comprising lead dioxide or manganese dioxide, andan inorganic semiconductor comprising triiron tetraoxide. Thesesemiconductors may be used individually or in combination of two or morethereof.

In the case where the other electrode is solid, an electrical conductinglayer may be provided thereon so as to attain good electrical contactwith an exterior leading line (for example, a lead frame).

The electrical conducting layer can be formed, for example, by thesolidification of an electrically conducting paste, plating, vapordeposition of a metal, or formation of a heat-resistant electricallyconducting resin film. Preferred examples of the electrically conductingpaste include silver paste, copper paste, aluminum paste, carbon pasteand nickel paste, and these may be used individually or in combinationof two or more thereof. In the case of using two or more kinds ofpastes, the pastes may be mixed or may be superposed one on another asseparate layers. The electrically conducting paste applied is thensolidified by allowing it to stand in air or by heating. Examples of theplating include nickel plating, copper plating, silver plating andaluminum plating. Examples of the metal vapor-deposited includealuminum, nickel, copper and silver.

More specifically, for example, carbon paste and silver paste arestacked in this order on the other electrode and these are sealed with amaterial such as epoxy resin, thereby constructing a capacitor. Thiscapacitor may have a niobium or tantalum lead which is sintered andmolded integrally with the niobium sintered body or welded afterward.

In the case where the other electrode is liquid, the capacitorconstructed by the above-described two electrodes and the dielectricmaterial is housed, for example, in a can electrically connected to theother electrode to form a capacitor. In this case, the electrode side ofthe niobium sintered body is guided outside through the niobium ortantalum lead and at the same time, insulated from the can using aninsulating rubber or the like.

The capacitor having such a construction of the present invention isjacketed using, for example, resin mold, resin case, metallic jacketcase, dipping of resin or laminate film, and then used as a capacitorproduct for various uses.

When the capacitor of the present invention is used, a more compactcapacitor product can be obtained as compared with conventionalcapacitors having the same voltage resistance and the same capacitance.

In the electronic circuit of portable telephone, personal computer andthe like, many capacitors are used and when the capacitor of the presentinvention is used, the electronic circuit can be housed in a smallerspace than in the case of using conventional capacitors. In addition, byusing the capacitor of the present invention, an electronic instrumentmore compact than conventional ones can be obtained.

MODE FOR CARRYING OUT THE INVENTION

The present invention is described in greater detail below by referringto Examples and Comparative Examples.

The nitrogen content of niobium powder was determined using anitrogen/oxygen analyzer manufactured by LECO and the Cr content wasmeasured by IPC-MS in each Example and Comparative Example.

The voltage resistance value of the produced capacitor was designated asa voltage value when a voltage was applied to 30 units of capacitors ineach Test Example while elevating in sequence by 1 V and the number ofshort-circuited capacitors exceeded 5 units.

EXAMPLES 1 to 7 AND COMPARATIVE EXAMPLE

A niobium powder (average particle size: 3 μm) obtained by introducing ahydrogen gas into a niobium ingot and wet-cracking the ingot waspulverized in a jet mill in a nitrogen atmosphere without passingthrough dehydrogenation. The pulverized niobium powder was not taken outoutside but left standing at first at 400° C. under reduced pressure fordehydrogenation, further left standing at 850° C. and then cracked toproduce a niobium powder. Subsequently, a nitrogen gas was passedtherethrough at 300° C. for 20 minutes to obtain 100 g of a partially(about 1,600 ppm by mass) nitrided niobium powder.

10 g of niobium powder at this stage was used for Comparative Exampleand the remaining 90 g was dipped in a 3:2 mixed solution of nitric acidand aqueous hydrogen peroxide and stirred at room temperature. About 10g of niobium powder was extracted every one-hour stirring and theniobium powder in each portion was washed with pure water until the pHof the washing water reached 7 and then dried in a vacuum to obtain 10 gof niobium powder for each of Examples 1 to 7. The average particle sizeand the Cr content of each niobium powder are shown in Table 1.

TABLE 1 Average Particle Size of Cr Content, Niobium Powder, μm mass ppmComparative 0.9 65 Example Example 1 0.8 49 Example 2 0.8 35 Example 31.0 19 Example 4 0.9 8 Example 5 1.0 5 Example 6 0.9 0.8 Example 7 0.90.5

Subsequently, using the niobium powder of each Example, 30 units ofmolded articles having a size of 1.8 mm×3.5 mm×4.5 mm were produced. Atthis time, a niobium wire having a diameter of 0.3 mm was integrallymolded to work out to a lead. These molded articles were sintered at1,250° C. in a vacuum of 7×10⁻³ Pa to obtain sintered bodies. Eachsintered body was electrochemically formed in an aqueous 0.1% phosphoricacid solution at a temperature of 80° C. and 12 V to form a dielectriclayer comprising niobium oxide. Thereafter, polypyrrole (using ammoniumpersulfate as an oxidant and sodium anthraquinone-sulfonate as a dopant,a reaction between pyrrole and the oxidant was repeated in the presenceof a dopant) was filled in pores inside the sintered body as materialfor the other electrode. Furthermore, carbon paste and silver paste werestacked in this order and after mounting on a lead frame, the whole wassealed with an epoxy resin to produce a capacitor.

The sintered bodies in respective Examples all had a specific surfacearea of 1 m²/g. The capacitance and the voltage resistance of thecapacitors produced are shown in Table 2.

TABLE 2 Number of short- circuited capacitors Voltage when a voltage ata Capacitance Resistance voltage resistance μF V was applied Comparative800 5 6 Example Example 1 820 6 7 Example 2 830 8 25 Example 3 800 8 20Example 4 810 8 18 Example 5 810 8 16 Example 6 830 8 7 Example 7 820 86

It is seen from the results of Examples 1 to 7 that as the Cr content inniobium powder becomes less, the capacitor produced from the niobiumpowder can have better voltage resistance characteristics.

INDUSTRIAL APPLICABILITY

A capacitor having good voltage resistance characteristics can beobtained by using the niobium for capacitors of the present inventionwhich contains small amount of chromium.

1. A capacitor constructed by one electrode formed of a niobium sinteredbody mainly comprising niobium, the other electrode and a dielectricmaterial interposed between the two electrodes, wherein the sinteredbody is a sintered body mainly comprising niobium having a chromiumcontent of from 0.8 ppm by mass or less.
 2. The capacitor according toclaim 1, wherein the niobium contains a niobium nitride.
 3. Thecapacitor according to claim 1, wherein the niobium contains a niobiumcarbide.
 4. The capacitor according to claim 1, wherein the niobiumcontains a niobium boride.
 5. The capacitor according to claim 1,wherein the niobium is a powder having an average particle size of 0.1μm to 3 μm.
 6. The capacitor according to claim 1, wherein the niobiumis a niobium-granulated product having an average particle size of 10 μmto 300 μm.
 7. A capacitor constructed by one electrode formed of aniobium sintered body mainly comprising niobium, the other electrode anda dielectric material interposed between the two electrodes, wherein thesintered body mainly comprising niobium having a chromium content from0.8 ppm by mass or less and wherein the niobium sintered body has a BETspecific surface area from 0.5 m2/g to 7 m2/g.
 8. The capacitor asclaimed in claim 1, wherein the main component of the dielectricmaterial constituting the capacitor is a niobium oxide.
 9. The capacitoras claimed in claim 1, wherein the other electrode is at least onemember selected from the group consisting of an electrolytic solution,an organic semiconductor and an inorganic semiconductor.
 10. Thecapacitor as claimed in claim 9, wherein the organic semiconductor is atleast one organic semiconductor selected from the group consisting of anorganic semiconductor comprising a benzopyrroline tetramer andchloranile, an organic semiconductor mainly comprisingtetrathiotetracene, an organic semiconductor mainly comprisingtetracyanoquinodimethane and an organic semiconductor mainly comprisingan electrically conducting polymer obtained by doping a dopant into apolymer comprising two or more repeating units represented by thefollowing formula (1) or (2):

(wherein R₁ to R₄ each represents a monovalent group selected from thegroup consisting of a hydrogen atom, a linear or branched, saturated orunsaturated alkyl, alkoxy or alkylester group having from 1 to 10 carbonatoms, a halogen atom, a nitro group, a cyano group, a primary,secondary or tertiary amino group, a CF₃ group, a phenyl group and asubstituted phenyl group; the hydrocarbon chains in each of the pairs R₁and R_(2,) and R₃ and R₄ may combine at an arbitrary position to form adivalent chain for forming at least one 3-, 4-, 5-, 6- or 7-memberedsaturated or unsaturated hydrocarbon cyclic structure together with thecarbon atoms substituted by R₁ and R₂ or by R₃ and R₄; the cyclic bondedchain may contain a bond of carbonyl, ether, ester, amide, sulfide,sulfinyl, sulfonyl or imino at an arbitrary position; X represents anoxygen atom, a sulfur atom or a nitrogen atom; R₅ is present only when Xis a nitrogen atom and each R₅ independently represents hydrogen or alinear or branched, saturated or unsaturated alkyl group having from 1to 10 carbon atoms).
 11. The capacitor as claimed in claim 10, whereinthe electrically conducting polymer is an electrically conductingpolymer comprising a repeating unit represented by the following formula(3):

(wherein R₆ and R₇ each independently represents a hydrogen atom, alinear or branched, saturated or unsaturated alkyl group having from 1to 6 carbon atoms, or a substituent for forming at least one 5-, 6- or7-membered saturated hydrocarbon cyclic structure containing two oxygenelements resulting from those alkyl groups combining with each other atan arbitrary position, and the cyclic structure includes a structurehaving a vinylene bond which may be substituted, and a phenylenestructure which may be substituted).
 12. The capacitor as claimed inclaim 9, wherein the organic semiconductor is at least one memberselected from the group consisting of polypyrrole, polythiophene,polyaniline and substitution derivatives thereof.
 13. The capacitor asclaimed in claim 9, wherein the organic or inorganic semiconductor hasan electrical conductivity of 10⁻² S·cm⁻¹ to 10³ S·cm⁻¹.
 14. A methodfor producing the capacitor claimed in claim 8, wherein the niobiumoxide is formed by electrolytic oxidation.
 15. An electronic circuitusing the capacitor claimed in claim
 1. 16. An electronic instrumentusing the capacitor claimed in claim
 1. 17. A method for producing acapacitor constructed by one electrode mainly comprising niobium, theother electrode and a dielectric material interposed between the twoelectrodes, comprising a step of reducing the chromium content in theelectrode mainly comprising niobium wherein the chromium content is from0.8 or less.
 18. The method for producing a capacitor according to claim17, wherein the step of reducing the chromium content is a step oftreating a substance mainly comprising niobium with a solutioncontaining at least one acid selected from the group consisting of ahydrofluoric acid, a nitric acid, a sulfuric acid and a hydrochloricacid.
 19. The method for producing a capacitor according to claim 17,wherein the niobium contains a niobium nitride.
 20. The method forproducing a capacitor according to claim 17, wherein the niobiumcontains a niobium carbide.
 21. The method for producing capacitoraccording to claim 17, wherein the niobium contains a niobium boride.22. The method for producing capacitor according to claim 17, whereinthe niobium is a powder.
 23. The method for producing capacitoraccording to claim 22, wherein the niobium powder has an averageparticle size of 0.1 μm to 3 μm.
 24. The method for producing capacitoraccording to claim 17, wherein the niobium is a niobium granulatedproduct having an average particle size of 10 μm to 300 μm.
 25. Themethod for producing capacitors according to claim 17, wherein theniobium is a niobium sintered body having a BET specific surface area of0.5 m²/g to 7 m²/g.
 26. A method for producing a capacitor according toclaim 17, further comprising granulating the niobium.
 27. A method forproducing a capacitor according to claim 26, further comprising sinteredthe granulated niobium.
 28. A capacitor obtained by the productionmethod claimed in claim 17.